1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular relates to replenishment control for maintaining a toner density in a developing unit at a target density.
2. Description of the Related Art
A developing unit using a two-component developer including a toner and a carrier detects toner density by a sensor to maintain toner density at a target density (Japanese Patent Laid-Open No. H08-110696). When toner is used for an image formation, the toner is replenished from a toner tank to the developing unit, and the toner and the carrier are mixed by a mixer.
In recent years, there is a demand for miniaturization, a reduction in capacity or the like in developing units. If a developing unit is miniaturized, the amount of replenished toner per time increases with respect to the capacity of the developing unit, and there are cases in which the toner and the carrier are not mixed sufficiently. In particular, toner density outputted by a sensor tends to fluctuate immediately after the toner is replenished. This is especially noticeable for a small-scale developing unit. An output value of the sensor repeatedly increases/decreases and finally converges to the actual toner density. Accordingly, if the toner is replenished using toner density acquired from the sensor when the toner and the carrier are not mixed sufficiently, the toner density cannot be controlled to the target density.